Hydraulically operated continuously variable transmission

ABSTRACT

A hydraulically operated continuously variable transmission includes a hydraulic motor coupled to an output shaft, a hydraulic pump coupled to an input shaft, and a closed hydraulic circuit interconnecting the hydraulic motor and the hydraulic pump. The hydraulic motor has an annular array of slidable motor plungers disposed around an axis of rotation thereof and slidably disposed in respective cylinder holes defined in a motor cylinder, the motor cylinder having an annular array of communication ports disposed around the axis of rotation and communicating with the cylinder holes. The hydraulic motor also has a distribution ring supported on a fixed shaft and rotatable relative to and slidable against an end face of the motor cylinder, the distribution ring having a suction port for communicating with the communication ports corresponding to those motor plungers which are in an expansion stroke, and a discharge port for communicating with the communication ports corresponding to those motor plungers which are in a contraction stroke. The suction and discharge ports are arcuate in shape and extend substantially along the annular array of communication ports, the suction and discharge ports having outer peripheral edges having a radius greater than the radius of a circumscribed circle of the communication ports and inner peripheral edges having a radius smaller than the radius of an inscribed circle of the communication ports.

BACKGROUND OF THE INVENTION

The present invention relates to a hydraulically operated continuously variable transmission including a hydraulic pump and a hydraulic motor which are interconnected by a closed hydraulic circuit, and more particularly to a distribution ring for distributing oil discharged from a hydraulic pump to a hydraulic motor in a hydraulically operated continuously variable transmission.

There have been proposed various hydraulically operated continuously variable transmissions for use in automobiles, including a swash-plate axial-plunger hydraulic pump having a pump cylinder coupled to an input shaft and a plurality of pump plungers slidably disposed in respective cylinder holes defined in the pump cylinder in an annular pattern around an axis of rotation of the pump cylinder, the hydraulic pump having an outlet port, a swash-plate axial-plunger hydraulic motor having a motor cylinder coupled to an output shaft and a plurality of motor plungers slidably disposed in respective cylinder holes defined in the motor cylinder in an annular pattern around an axis of rotation of the motor cylinder, the hydraulic motor having an inlet port, and a closed hydraulic circuit interconnecting the hydraulic pump and the hydraulic motor. Power is transmitted by the transmission through relative rotation between the motor cylinder and the pump cylinder.

As disclosed in Japanese Patent Publications Nos. 32-7159 and 41-3208, for example, communication ports arranged at substantially equal angular intervals in communication with the respective cylinder holes in the motor cylinder are defined in the motor cylinder and open at an axial end face thereof in an annular pattern around the axis of rotation of the motor cylinder. An annular distribution ring is disposed eccentrically with respect to the axis of rotation of the motor cylinder and held in slidable contact with the end face of the motor cylinder. Oil discharged from the hydraulic pump is introduced into the distribution ring, whereas oil discharged from the motor cylinder is introduced around the distribution ring.

The outlet and inlet ports of the pump are brought into successive communication with the communication ports in the motor cylinder in response to relative rotation between the distribution ring and the motor cylinder for reciprocally moving the annularly arranged motor plungers.

In the above structure, the communication ports are divided into a group of communication ports in an expansion stroke and a group of communication ports in a contraction stroke by the annular distribution ring which is disposed eccentrically with respect to the center of the pitch circle of the communication ports. Therefore, it would be difficult to provide a large open area over a wide angle in the direction of rotation. Particularly when the hydraulic motor rotates at a high speed, the resistance to suction is excessively large in a suction stroke, thus generating cavitation, and the resistance to discharge is excessively large in a discharge stroke, thus producing a pressure loss. At any rate, the conventional arrangement is responsible for lowering the efficiency of operation of the hydraulic motor.

SUMMARY OF THE INVENTION

In view of the aforesaid drawback of the prior art, it is an object of the present invention to provide a hydraulically operated continuously variable transmission which prevents the operation efficiency of a hydraulic motor from being lowered even when the hydraulic motor operates at a high speed.

According to the present invention, the above object can be accomplished by a hydraulically operated continuously variable transmission comprising a transmission case, a fixed shaft mounted in the transmission case, an output shaft rotatably supported in the transmission case, a hydraulic motor coupled to the output shaft and having a motor cylinder and an annular array of slidable motor plungers disposed around an axis of rotation thereof and slidably disposed in respective cylinder holes defined in the motor cylinder, the motor cylinder having an annular array of communication ports disposed around the axis of rotation and communicating with the cylinder holes, the communication ports opening at an end face of the motor cylinder, the hydraulic motor also having a distribution ring supported on the fixed shaft and rotatable relatively to and slidable against the end face of the motor cylinder, the distribution ring having a suction port for communicating with the communication ports corresponding to those motor plungers which are in an expansion stroke, and a discharge port for communicating with the communication ports corresponding to those motor plungers which are in a contraction stroke, the suction and discharge ports being arcuate in shape and extending substantially along the annular array of communication ports, the suction and discharge ports having outer peripheral edges having a radius greater than the radius of a circumscribed circle of the communication ports and inner peripheral edges having a radius smaller than the radius of an inscribed circle of the communication ports, an input shaft rotatably supported in the transmission case, a hydraulic pump coupled to the input shaft, at least one of the hydraulic pump and the hydraulic motor being of the variable-displacement type, and a closed hydraulic circuit interconnecting the hydraulic pump and the hydraulic motor.

With the above arrangement, the communication ports and the suction and discharge ports of the distribution ring can be registered with each other through an increased area, thus reducing the resistance to suction and discharge of the working oil therethrough.

The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a hydraulically operated continuously variable transmission according to the present invention;

FIG. 2 is an end view of a distribution ring;

FIG. 3 is a cross-sectional view taken along line III--III of FIG. 2;

FIG. 4 is a schematic view showing the relationship between the distribution ring and communication ports;

FIG. 5 is a fragmentary perspective view, partly cut away, of the distribution ring which is mounted on a fixed shaft;

FIG. 6 is a view similar to FIG. 2, showing a distribution ring according to another embodiment of the present invention; and

FIG. 7 is a cross-sectional view taken along line VII--VII of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a hydraulically operated continuously variable transmission for use in a motor vehicle such as an automobile according to the present invention, the transmission basically comprising a hydraulic pump P and a hydraulic motor M housed in a transmission case 1 composed of a pair of longitudinally separated case members la, lb.

The hydraulic pump P has a pump cylinder 4 splined to an end 3 of an input shaft 2, a plurality of cylinder holes or bores 5 defined in the pump cylinder 4 in a circular pattern around and concentric with the input shaft 2 and extending parallel to the input shaft 2, and a plurality of pump plungers 6 slidably fitted respectively in the cylinder holes 5. The hydraulic pump P can be driven by the power of an engine (not shown) which is transmitted through a flywheel 7 coupled to the opposite end of the input shaft 2.

The hydraulic motor M has a motor cylinder 8 disposed in surrounding relation to the pump cylinder 4, a plurality of cylinder holes or bores 9 defined in the motor cylinder 8 in a circular pattern around and concentric with the input shaft 2 and extending parallel to the input shaft 2, and a plurality of motor plungers 10 slidably fitted respectively in the cylinder holes 9. The hydraulic motor M is rotatable relatively to the pump cylinder 4 in concentric relation thereto.

The motor cylinder 8 has axially opposite ends on which a pair of support shafts lla, llb is disposed, respectively. The support shaft lla is rotatably supported on the axial end wall of the case member lb by means of a ball bearing 12, and the support shaft 11b is rotatably supported on the axial end wall of the case member 1a by means of a needle bearing 13. A holder plate 14 is fixed by bolts 15 to the axial end wall of the case member 1b. The ball bearing 12 and the support shaft lla are thus fixedly mounted on the case member lb against axial movement. The other support shaft llb has an integral spur gear 16 for transmitting output power of the hydraulic motor M through a differential gear mechanism (not shown) to an outside member.

A pump swash plate 17 inclined at an angle to the pump plungers 6 is fixedly disposed radially inwardly of the motor cylinder 8. An annular pump shoe 18 is rotatably slidably supported on an inclined surface of the pump swash plate 17.

Each of the pump plungers 6 has a bottomed hole 19 opening toward the pump swash plate 17. A connecting rod 20 inserted in the bottomed hole 19 is pivotally movable with respect to the pump plunger 6 by means of a ball joint 21a on the inner end of the connecting rod 20. The connecting rod 20 projects out of the corresponding pump plunger 6 from the bottomed hole 19, and is pivotally movable with respect to the pump shoe 18 by means of a ball joint 21b on the outer projecting end of the connecting rod 20.

The annular pump shoe 18 has its outer peripheral surface supported in the motor cylinder 8 by a needle bearing 22. The annular pump shoe 18 has an annular step 23 defined in its inner peripheral surface facing the pump plungers 6. A presser ring 24 riding in the annular step 23 presses the pump shoe 18 toward the pump swash plate 17 under the resiliency of a compression coil spring 26 disposed under compression around the input shaft 2 and acting on a spring holder 25 held against the presser ring 24. The spring holder 25 is slidably fitted over splines 27 on the input shaft 2, and has a partly spherical surface contacting a complementary partly spherical surface of the presser ring 24. Therefore, the spring holder 25 is neatly held against the presser ring 24 for transmitting the resilient force from the spring 26 to the presser ring 24 irrespective of how the spring holder 25 and the presser ring 24 are relatively positioned.

Thus, the pump shoe 18 can be slidingly rotated in a fixed position on the pump swash plate 17 at all times.

The pump shoe 18 has a crown gear 28 on the end face thereof facing the pump cylinder 4, the crown gear 28 extending around the outer periphery of the pump shoe 18. A bevel gear 29, which has the same number of teeth as the crown gear 28, is fixed to the outer periphery of the pump cylinder 4, and held in mesh with the crown gear 28. When the pump cylinder 4 is driven to rotate by the input shaft 2, the pump shoe 18 is rotated in synchronism with the pump cylinder 4 through the meshing gears 28, 29. On rotation of the pump shoe 18, those pump plungers 6 which run along an ascending side of the inclined surface of the pump swash plate 17 are moved in a discharge stroke by the pump swash plate 17, the pump shoe 18, and the connecting rods 20, and those pump plungers 6 which travel along a descending side of the inclined surface of the pump swash plate 17 are moved in a suction stroke.

A needle bearing 30 is disposed between the outer peripheral surface of the bevel gear 29 and the inner peripheral surface of the motor cylinder 8. Therefore, concentric relative rotation of the pump cylinder 4 and the motor cylinder 8 is performed with increased accuracy.

The pump shoe 18 has hydraulic pockets 31 defined in its surface held against the pump swash plate 17 and positioned in alignment with the respective connecting rods 20. The hydraulic pockets 31 communicate with the respective oil chambers in the pump cylinder 4 through oil holes 32 defined in the pump plungers 6, oil holes 33 defined in the connecting rods 20, and oil holes 34 defined in the pump shoe 18. While the pump cylinder 4 is in operation, therefore, oil under pressure in the pump cylinder 4 is supplied to the hydraulic pockets 31 to apply a hydraulic presssure to the pump shoe 18 in a direction to bear the thrust force imposed by the pump plungers 6 on the pump shoe 18. Therefore, the oil supplied to the hydraulic pockets 31 serves to reduce the pressure under which the pump shoe 19 contacts the pump swash plate 17, and also to lubricate the mutually sliding surfaces of the pump shoe 18 and the pump swash plate 17.

A motor swash plate 35 is tiltably supported in the transmission case 1 by means of a pair of trunnions 36 projecting from opposite sides of the motor swash plate 35. The motor swash plate 35 is held in confronting relation to the motor plungers 10. The motor swash plate 35 has an inclined surface on which there is slidably disposed a motor shoe 37 that is pivotally coupled to ball joints 38 on the outer ends of the motor plungers 10.

Each of the motor plungers 10 reciprocally moves in expansion and compression strokes while rotating the motor cylinder 8. The stroke of the motor plungers 10 can continuously be adjusted from zero to a maximum level by varying the angle of inclination of the motor swash plate 35 from a vertical position (shown by the two-dot-dash lines) in which the motor swash plate 35 lies perpendicularly to the motor plungers 10 to a most inclined position (shown by the solid lines).

The motor cylinder 8 comprises axially separate first through fourth members or segments 8a through 8d. The first member 8a includes the support shaft llb and accommodates the pump swash plate 17. The second member 8b has guide holes in the cylinder holes 9, in which the motor plungers 10 are slidably guided, respectively. The third and fourth members 8c, 8d have oil chambers 39 in the cylinder holes 9, the oil chambers 39 being slightly larger in diameter than the guide holes in the cylinder holes 9. The third member 8c serves as a distribution member 40 having oil passages leading to the cylinder holes 5, 9, and the fourth member 8d includes the support shaft lla. The first through fourth members 8a-8d are relatively positioned by knock pins, for example, inserted in their mating end faces, and are firmly coupled together by means of a plurality of bolts 41a , 41b.

The input shaft 2 has an outer end portion rotatably supported centrally in the support shaft 11b of the motor cylinder 8 by a needle bearing 42, and an inner end portion rotatably supported centrally in the distribution member 40 by a needle bearing 43.

The spring 26 is disposed under compression between the pump cylinder 4 and the spring holder 25 for pressing the pump cylinder 4 against the distribution member 40 to prevent oil from leaking from between the sliding surfaces of the pump cylinder 4 and the distribution member 40. The resilient force of the spring 26 is also effective in supporting the spring holder 25, the presser ring 24, the pump shoe 18, and the pump swash plate 17 firmly in the motor cylinder 8, as described above.

The support shaft lla is of a hollow structure in which a fixed shaft 44 is centrally inserted. A distribution ring 45 is fitted over the inner end of the fixed shaft 44 in a fluid-tight manner through an 0-ring therebetween. The distribution ring 45 has an axial end face held in sliding contact with the distribution member 40. The fourth member 8d of the motor cylinder 8 has an interior hollow space 46 which is divided by the distribution ring 45 into an inner oil chamber 46a and an outer oil chamber 46b.

The distribution member 40 has an outlet port 47 and an inlet port 48. The outlet port 47 provides fluid communication between the cylinder holes 5 that receive the pump plungers 6 operating in the discharge stroke and the inner oil chaber 46a. The inlet port 48 provides fluid communication between the cylinder holes 5 that receive the pump plungers 6 operating in the suction stroke and the outer oil chamber 46b. The distribution member 40 also has a number of communication ports 49 defined therein and through which the cylinder holes 9 of the motor cylinder 8 communicate with the interior space 46 in the fourth member 8d.

The communication ports 49 open into the interior space 46 at equally spaced locations on a circle around the axis of rotation of the hydraulic motor M. In response to rotation of the motor cylinder 8, the communication ports 49 are caused by the distribution ring 46 slidingly held against the distribution member 40 to successively communicate with the inner and outer oil chambers 46a, 46b.

Therefore, a closed hydraulic circuit is formed between the hydraulic pump P and the hydraulic motor M through the distribution member 40 and the distribution ring 45. When the pump cylinder 4 is driven by the input shaft 2, high-pressure working oil discharged by the pump plungers 6 in the discharge stroke flows from the outlet port 47, the inner oil chamber 46a, and the communication ports 49 communicating with the inner oil chamber 46a into the cylinder holes 9 receiving the motor plungers 10 which are in the expansion stroke, thereby imposing a thrust on these motor plungers 10.

Working oil discharged by the motor plungers 10 operating in the compression stroke flows through the communication ports 49 communicating with the outer oil chamber 46b and the inlet port 48 into the cylinder holes 5 receiving the pump plungers 6 in the suction stroke. Upon such circulation of the working oil, the motor cylinder 8 is driven by the sum of the reactive torque applied by the pump plungers 6 in the discharge stroke to the motor cylinder 8 through the pump swash plate 17 and the reactive torque received by the motor plungers 10 in the expansion stroke from the motor swash plate 35.

The transmission ratio of the motor cylinder 8 to the pump cylinder 4 is given by the following equation: ##EQU1##

It can be understood from the above equation that the transmission ratio can be varied from 1 to a desired value by varying the displacement of the hydraulic motor M from zero to a certain value.

Since the displacement of the hydraulic motor M is determined by the stroke of the motor plungers 10, the transmission ratio can continuously be adjusted from 1 to a certain value by tilting the motor swash plate 35 from the vertical position to a certain inclined position.

A hydraulic ratio-changing servomotor Sl for tilting the motor swash plate 35 is disposed in an upper portion of the transmission case 1. The ratio-changing servomotor Sl has a piston rod 50 having an end projecting into the transmission case 1. The projecting end of the piston rod 50 is coupled to the motor swash plate 35 through a connector 51 and pivot pins. The servomotor Sl has a pilot valve 52, and the outer end of the pilot valve 52 projecting through the holder plate 14 is coupled to a cam mechanism Cl. The motor swash plate 35 is remotely controlled by a control device (not shown) through the servomotor Sl and the cam mechanism Cl.

The ratio-changing servomotor Sl is of the known type in which a piston therein is reciprocally operated in amplified movement by following the movement of the pilot valve 52 which is given by the control device. In response to operation of the servomotor Sl, the motor swash plate 35 can continuously be angularly shifted or adjusted from the most inclined position indicated by the solid lines in FIG. 1 where the transmission ratio is maximum to the least inclined position indicated by the imaginary (two-dot-and-dash) lines where the transmission ratio is minimum.

A stopper 53 is interposed between the motor swash plate 35 and the end wall of the case member la for limiting the mechanical least inclined position of the motor swash plate 35.

The fixed shaft 44 is of a hollow construction having a peripheral wall having radial connecting ports 54a, 54b through which the inner and outer oil chambers 46a, 46b communicate with each other. A cylindrical clutch valve 55 is fitted in the interior space of the fixed shaft 44 for selectively opening and closing the ports 54a, 54b, the clutch valve 55 being rotatable relative to the fixed shaft 44 through a needle bearing 56. The clutch valve 55 serves as a clutch for selectively connecting and disconnecting the hydraulic pump P and the hydraulic motor M. The clutch valve 55 is operatively coupled to a clutch control unit (not shown). When the ports 54a, 54b are fully opened, the clutch is in an "OFF" position. When the ports 54a, 54b are partly opened, the clutch is in a "partly ON" position. When the ports 54a, 54b are fully closed, the clutch is in an "ON" position. With the clutch OFF as shown, working oil discharged from the outlet port 47 into the inner oil chamber 46a flows through the ports 54a, 54b and the outer oil chamber 46b directly into the inlet port 48, making the hydraulic motor M inoperative. When the clutch is ON, the above oil flow is shut off, and working oil is circulated from the hydraulic pump P to the hydraulic motor M, allowing hydraulic power to be transmitted from the hydraulic pump P to the hydraulic motor M.

A servomotor S2 for selectively making and breaking the hydraulic circuit is disposed centrally in the hollow clutch valve 55. The servomotor S2 is operatively coupled to the ratio-changing servomotor Sl through the cam mechanism Cl. When a pilot valve 57 of the servomotor S2 which projects out from the holder plate 14 is pushed, a shoe 58 on the distal end of the servomotor S2 closes the open end of the outlet port 47 in the distribution member 40, thereby cutting off the flow of working oil from the outlet port 47 into the inner oil chamber 46a.

With the oil flow thus cut off, the pump plungers 6 are hydraulically locked and the hydraulic pump P and the hydraulic motor M are directly connected to each other, so that the motor cylinder 8 can mechanically be driven by the pump cylinder 4 through the pump plungers 6 and the pump swash plate 17. The hydraulic pump P and the hydraulic motor M are directly interconnected in this manner when the motor swash plate 35 is vertically positioned for the minimum transmission ratio. In this transmission position, the efficiency of transmission of power from the input shaft to the output shaft is increased, and the thrust applied by the motor plungers 10 to the motor swash plate 35 is reduced, thus lessening the stresses on the bearings and other members.

The cam mechanism Cl, the holder plate 14, and other members are covered with an end cover 59 attached to the righthand side end of the transmission case 1.

As shown in FIGS. 2 and 3, the distribution ring 45 comprises a disc portion 45a held in slidable contact with the end face of the distribution member 40, and a cylindrical portion 45b receiving the free end of the fixed shaft 44. The disc portion 45a has defined therein a central hole 60 communicating with the outlet port 47 of the hydraulic pump P and through which the shoe 58 of the servomotor S2 can be inserted, a suction port 61 providing communication between the end face of the disc portion 45a which is held in slidable contact with the distribution member 40 and the interior of the cylindrical portion 45b, and a discharge port 62 providing communication between the end face of the disc portion 45a which is held in slidable contact with the distribution member 40 and the exterior of the cylindrical portion 45b.

The suction port 61 communicates the inner oil chamber 46a with the communication ports 49 corresponding to those cylinder holes in the expansion stroke, whereas the discharge port 62 communicates the outer oil chamber 46b with the communication ports 49 corresponding to those cylinder holes in the contraction stroke. As shown in FIG. 4, the ports 61, 62 are arcuate in shape and extend substantially along the pitch circle of the annular pattern of the communication ports 49 so that the ports 61, 62 can communicate with plural communication ports 49 simultaneously. The ports 61, 62 have inner peripheral edges having a radius Ri smaller than the radius Rl of an inscribed circle of the communication ports 49, and outer peripheral edges having a radius Ro larger than the radius R2 of a circumscribed circle of the communication ports 49. The arcs of the ports 61, 62 being thus determined, a sufficient open area of the communication ports 49 is provided for reducing the resistance to flow of the working oil therethrough.

The high-pressure oil which is discharged by the pump plungers 6 in the discharge stroke to flow from the discharge port 47 into the inner oil chamber 46a flows via the suction port 61 and those communication ports 49 which are in registry with the suction port 61 into the cylinder holes 9 of the hydraulic motor M. At this time, as shown in FIG. 4, the communication ports 49 as they rotate are registered alternately with the suction and discharge ports 61, 62. Those communication ports 49 which communicate with the motor plungers 10 that start to move in the expansion stroke are supplied with high-pressure oil while overlapping the upstream end of the suction port 61 in the direction of rotation. As the motor plungers 10 are moved out of the expansion stroke, the supply of high-pressure oil is cut off while overlapping the downstream end of the suction port 61. When the motor plungers 10 enter the contraction stroke, oil is discharged from the upstream end of the discharge port 62 in the direction of rotation, and when the motor plungers 10 are moved out of the contraction stroke, the discharge of oil is stopped from the downstream end of the discharge port 62.

As shown in FIG. 5, a pair of diametrically opposite fingers 63 projects axially from the end of the cylinderical portion 45b of the distribution ring 45 remotely from the disc portion 45a and is disposed on the outer circumference of the cylindrical portion 45b. The outer circumferential surface of the fixed shaft 45 has a pair of diametrically opposite recesses 64 defined therein and receiving the fingers 63, respectively. With the fingers 63 engaging in the corresponding recesses 64, the cylindrical portion 45b is fitted in a fluid-tight manner over the axial end of the fixed shaft 44 with a seal member 65 such as an 0-ring therebetween. Thus, the distribution ring 45 and the fixed shaft 44 are prevented from being relatively angularly displaced by the fingers 63 received in the recesses 64.

FIGS. 6 and 7 illustrate a distribution ring according to another embodiment of the present invention. In this embodiment, the suction port 61 has its inner peripheral edge cut off so as to be joined to the central hole 60, and the discharge port 62 has its outer peripheral edge cut off. The radius Ro of the outer peripheral edge of the suction port 61 and the radius Ri of the inner peripheral edge of the discharge port 62 are selected in the same manner as in the previous embodiment. With this arrangement, the ratio between the areas of the pressure bearing surfaces can appropriately be adjusted, and hence the balancing between the pressures acting on the sliding surfaces of the distribution member 40 and the distribution ring 45 can suitably be established.

With the present invention, as described above, the area through which the suction and discharge ports of the distribution ring and the communication ports of the distribution member are in registry with each other can be increased through a simple structure, for thereby reducing the resistance to flow of the working oil. Therefore, the operation efficiency of the hydraulic motor is increased, and so is the power transmission efficiency of the continuously variable transmission.

Although certain preferred embodiments have been shown and described, it should be understood that many changes and modifications may be made therein without departing from the scope of the appended claims. 

What is claimed is:
 1. A hydraulically operated continuously variable transmission comprising:a transmission case; a fixed shaft mounted in said transmission case; an output shaft rotatably supported in said transmission case; a hydraulic motor coupled to said output shaft and having a motor cylinder and an annular array of slidable motor plungers disposed around an axis of rotation thereof and slidably disposed in respective cylinder holes defined in said motor cylinder, said motor cylinder having an annular array of communication ports disposed around said axis of rotation and communicating with said cylinder holes, said communication ports opening at an end face of said motor cylinder, said hydraulic motor also having a distribution ring supported on said fixed shaft and rotatable relative to and slidable against said end face of said motor cylinder, said distribution ring having a suction port for communicating directly with the communication ports corresponding to those motor plungers which are in an expansion stroke, and a discharge port for communicating directly with the communication ports corresponding to those motor plungers which are in a contraction stroke, said suction and discharge ports being arcuate in shape and extending substantially along said annular array of communication ports, said suction and discharge ports having outer peripheral edges having a radius greater than the radius of a circumscribed circle of said communication ports and inner peripheral edges having a radius smaller than the radius of an inscribed circle of said communication ports; an input shaft rotatably supported in said transmission case; a hydraulic pump coupled to said input shaft, at least one of said hydraulic pump and said hydraulic motor being of the variable-displacement type; and a closed hydraulic circuit interconnecting said hydraulic pump and said hydraulic motor.
 2. A hydraulically operated continuously variable transmission according to claim 1, wherein said distribution ring has a central hole, said inner peripheral edge of the suction port being cut off with the suction port joined to said central hole, said outer peripheral edge of the discharge port being cut off.
 3. A fluid distribution apparatus in a hydraulically operated continuously variable transmission of the type having a hydraulic motor and a hydraulic pump connected through a closed hydraulic circuit, said hydraulic motor being disposed in a motor cylinder with a motor cylinder end face, said end face of said motor cylinder having an annular array of communication ports disposed around an axis of rotation of said motor cylinder for communicating with said hydraulic motor, comprising:a distribution ring for slidable engagement with said end face, said distribution ring being supported on a fixed shaft and rotatable relative to and slidable against said end face; a suction port in said distribution ring for communication with said communication ports when said communication ports are in a state to receive a working fluid, said suction port being slidable against said end face; and a discharge port in said distribution ring for communication with said communication ports when said communication ports are in a state to discharge said working fluid, said discharge port being slidable against said end face; said suction and discharge ports being arcuate in shape and extending substantially along said annular array of communication ports, said suction and discharge ports each having an outer peripheral edge having a radius greater than the radius of a circumscribed circle of said communication ports and an inner peripheral edge having a radius smaller than the radius of an inscribed circle of said communication ports.
 4. A fluid distribution apparatus according to claim 3, wherein said distribution ring has a cylindrical body engageable with said fixed shaft, said cylindrical body having an axis, said suction and discharge ports being eccentrically disposed with respect to the axis of said cylindrical body.
 5. A fluid distribution apparatus according to claim 3, wherein said distribution ring has a central hole, said inner peripheral edge of one of said suction port and said discharge port being cut off with the one of said suction port and said discharge port joined to said central hole, said outer peripheral edge of the other of said suction port and said discharge port being cut off.
 6. A fluid distribution apparatus according to claim 3, wherein said distribution ring has a central hole, said inner peripheral edge of the suction port being cut off with the suction port joined to said central hole, said outer peripheral edge of the discharge port being cut off.
 7. A fluid distribution apparatus in a hydraulically operated continuously variable transmission of the type having a hydraulic motor and a hydraulic pump connected through a closed hydraulic circuit, said hydraulic motor being disposed in a motor cylinder with a motor cylinder end face and having an annular array of slidable motor plungers disposed in respective cylinder holes defined in said motor cylinder, said end face of said motor cylinder having an annular array of communication ports disposed around an axis of rotation of said motor cylinder for communicating with said cylinder holes, comprising:a distribution ring with a distribution ring face for slidable engagement with said end face, said distribution ring being supported on a fixed shaft and rotatable relative to and slidable against said end face; a suction port in said distribution ring face for communication with the communication ports corresponding to those motor plungers which are in an expansion stroke; and a discharge port in said distribution ring face for communication with the communication ports corresponding to those motor plungers which are in a contraction stroke; said suction and discharge ports being arcuate in shape and extending substantially along said annular array of communication ports, said suction and discharge ports each having an outer peripheral edge having a radius greater than the radius of a circumscribed circle of said communication ports and an inner peripheral edge having a radius smaller than the radius of an inscribed circle of said communication ports.
 8. A hydraulically operated continuously variable transmission comprising:a transmission case; a fixed shaft mounted in said transmission case; an output shaft rotatably supported in said transmission case; a hydraulic motor coupled to said output shaft and having a motor cylinder and an annular array of slidable motor plungers disposed around an axis of rotation thereof and slidably disposed in respective cylinder holes defined in said motor cylinder, said motor cylinder having an annular array of communication ports disposed around said axis of rotation and communicating with said cylinder holes, said communication ports opening at an end face of said motor cylinder; a distribution ring supported on said fixed shaft and rotatable relative to and slidable against said end face of said motor cylinder, said distribution ring having a suction port slidable against said end face for communicating with the communication ports corresponding to those motor plungers which are in an expansion stroke, and a discharge port slidable against said end face for communicating with the Communication ports corresponding to those motor plungers which are in a contraction stroke, said suction and discharge ports being arcuate in shape and extending substantially along said annular array of communication ports, said suction and discharge ports each having an outer peripheral edge having a radius greater than the radius of a circumscribed circle of said communication ports and an inner peripheral edge having a radius smaller than the radius of an inscribed circle of said communication ports; an input shaft rotatably supported in said transmission case; a hydraulic pump coupled to said input shaft, at least one of said hydraulic pump and said hydraulic motor being of the variable-displacement type; and a closed hydraulic circuit interconnecting said hydraulic pump and said hydraulic motor.
 9. A hydraulically operated continuously variable transmission according to claim 8, wherein said distribution ring has a central hole, said inner peripheral edge of the suction port being cut off with the suction port joined to said central hole, said outer peripheral edge of the discharge port being cut off.
 10. A fluid distribution apparatus in a hydraulically operated continuously variable transmission of the type having a hydraulic motor and a hydraulic pump connected through a closed hydraulic circuit, said hydraulic motor being disposed in a motor cylinder with a motor cylinder end face, said end face of said motor cylinder having an annular array of communication ports disposed around an axis of rotation of said motor cylinder for communicating with said hydraulic motor, comprising:a distribution ring for slidable engagement with said end face, said distribution ring being supported on a fixed shaft and rotatable relative to and slidable against said end face; a suction port in said distribution ring for communication with said communication ports when said communication ports are in a state to receive a working fluid, said suction port being slidable against said end face; and a discharge port in said distribution ring for communication with said communication ports when said communication ports are in a state to discharge said working fluid, said discharge port being slidable against said end face; said suction and discharge ports being arcuate in shape and extending substantially along said annular array of communication ports, said suction and discharge ports each having an outer peripheral edge located radially outward of the radius of a circumscribed circle of said communication ports and an inner peripheral edge located radially inward of an inscribed circle of said communication ports.
 11. A fluid distribution apparatus according to claim 10, wherein said distribution ring has a central hole, said inner peripheral edge of one of said suction port and said discharge port being cut off with the one of said suction port and said discharge port joined to said central hole, said outer peripheral edge of the other of said suction port and said discharge port being cut off.
 12. A fluid distribution apparatus according to claim 10, wherein said distribution ring has a central hole, said inner peripheral edge of the suction port being cut off with the suction port joined to said central hole, said outer peripheral edge of the discharge port being cut off.
 13. A fluid distribution apparatus in a hydraulically operated continuously variable transmission of the type having a hydraulic motor and a hydraulic pump connected through a closed hydraulic circuit, said hydraulic motor being disposed in a motor cylinder with a motor cylinder end face, said end face of said motor cylinder having an annular array of communication ports disposed around an axis of rotation of said motor cylinder for communicating with said hydraulic motor, comprising:a distribution ring for slidable engagement with said end face, said distribution ring being supported on a fixed shaft and rotatable relative to and slidable against said end face, said distribution ring and said fixed shaft cooperating to form an interior region and an exterior region of said distribution ring; a suction port in said distribution ring for communication with said communication ports when said communication ports are in a state to receive a working fluid, said suction port being slidable against said end face; and a discharge port in said distribution ring for communication with said communication ports when said communication ports are in a state to discharge said working fluid, said discharge port being slidable against said end face; said suction and discharge ports being arcuate in shape and extending substantially along said annular array of communication ports, said suction and discharge ports each having an outer peripheral edge located radially outward of the radius of a circumscribed circle of said communication ports and an inner peripheral edge located radially inward of an inscribed circle of said communication ports.
 14. A fluid distribution apparatus according to claim 13, wherein one of said suction and said discharge ports communicates with the interior of said distribution ring and the other of said suction and said discharge ports communicates with the exterior of said distribution ring.
 15. A fluid distribution apparatus according to claim 13, wherein said suction port communicates with the interior of said distribution ring and said discharge port communicates with the exterior of said distribution ring.
 16. A fluid distribution apparatus according to claim 15, wherein said distribution ring has a central hole, said inner peripheral edge of the suction port being cut off with the suction port joined to said central hole, said outer peripheral edge of the discharge port being cut off. 